All Aboard!

Article 310, covering conductors for general wiring, has been totally rewritten for the 2011 National Electrical Code (NEC). The new format includes dividing the article into three parts: Part I covers general requirements; Part II covers installations of conductors; and Part III covers construction specifications for conductors. Extensive renumbering within the article has also occurred, with most of the ampacity tables being renumbered. While some of the changes within Article 310 are editorial in nature, with number and minor word changes, there are some very important changes that should be reviewed before applying any of the new requirements.

A new 310.2 has been added in Part I addressing two new definitions that are applied to 0- to 2,000-volt (V) insulated conductors, such as electrical ducts and thermal resistivity. An electrical duct is defined as “electrical conduits, or other raceways round in cross-section, that are suitable for use underground or imbedded in concrete.” As used in this Code, thermal resistivity is defined as “the heat transfer capability through a substance by conduction. It is the reciprocal of thermal conductivity and is designated “Rho” and expressed in the units “°C-cm/W.”

In new Part II for “installations” of conductors, 310.10 now covers “Uses Permitted” with the following text: “The conductors described in 310.104 shall be permitted for use in any of the wiring methods covered in Chapter 3 and as specified in their respective tables or as permitted elsewhere in this Code.”

All of the fine print notes were changed to informational notes in the 2011 NEC. The text from 310.13 in the 2008 NEC was relocated to the introductory text in 310.10 in the 2011 NEC as follows: “Informational Note: Thermoplastic insulation may stiffen at temperatures lower than –10°C (+14°F). Thermoplastic insulation may also be deformed at normal temperatures where subjected to pressure, such as at points of support. Thermoplastic insulation, where used on DC circuits in wet locations, may result in electroendosmosis between conductor and insulation.”
Many of the former sections, from 310.2 through 310.10 in the 2008 NEC, are lettered subsections of 310.10 in the 2011 NEC with the titles of these old sections retained within the new subsections. For example, former 310.8 covering conductors in wet, damp and dry locations and those exposed to direct sunlight are now 310.10(A), (B), (C) and (D), respectively, retaining the former titles with the information within these subsections primarily the same as in past editions of the NEC.

Section 310.6, covering shielding for higher voltage conductors, is now located in 310.10(E). It has been revised substantially with new 5,000V, nonshielded permissive text and an exception added. The underlined text denotes the new information: “(E) Shielding. Non-shielded, ozone-resistant insulated conductors with a maximum phase-to-phase voltage of 5000-volts shall be permitted in Type MC cables in industrial establishments where the conditions of maintenance and supervision ensure that only qualified persons service the installation. For other establishments, solid dielectric insulated conductors operated above 2,000-volts in permanent installations shall have ozone-resistant insulation and shall be shielded. All metallic insulation shields shall be connected to a grounding electrode conductor, a grounding busbar, an equipment grounding conductor, or a grounding electrode. Shielding shall be for the purpose of confining the voltage stresses to the insulation.”

An informational note has been added that explains the primary purpose of shielding and may help the reader understand the reasoning behind the shielding text as follows: “Informational Note: The primary purposes of shielding are to confine the voltage stresses to the insulation, dissipate insulation leakage current, drain off the capacitive charging current, and carry ground fault current to facilitate operation of ground fault protective devices in the event of an electrical cable fault.”

The new exception reads: “Exception No. 2: Nonshielded insulated conductors listed by a qualified testing laboratory shall be permitted for use up to 5000-volts to replace existing non-shielded conductors, on existing equipment in industrial establishments only, under the following conditions:
“Where the conditions of maintenance and supervision ensures that only qualified personnel install and service the installation.

A. “Conductors shall have insulation resistant to electric discharge and surface tracking, or the insulated conductor(s) shall be covered with a material resistant to ozone, electric discharge, and surface tracking.

B. “Where used in wet locations, the insulated conductor(s) shall have an overall nonmetallic jacket or a continuous sheath.

C. “Insulation and jacket thicknesses shall be in accordance with Table 310.13(D).

D. “Informational Note: Relocation or replacement may not comply with the term existing as related to this exception.”

Due to the complexity and extent of these changes, I will continue this review in the next issue.

ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.